Method and apparatus for determining location using phase difference of arrival

ABSTRACT

An ultra-wideband (“UWB”) communication system comprising a transmitter and a receiver having two antennas. An UWB signal transmitted by the transmitter is received at each of the antennas. By comparing the carrier phases of the received signals, the phase difference can be determined. From this phase difference and the known distance, d, between the antennas, the Cartesian (x, y) location of the transmitter relative to the receiver can be directly determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation application of U.S. patent application Ser. No. 16/352,372, filed 13 Mar. 2019 (“First Divisional Application”), which is a divisional of U.S. patent application Ser. No. 15,974,412, filed on 8 May 2018 and now issued as U.S. Pat. No. 10,509,116 (“First Parent Application”), which claims the benefit of U.S. provisional patent application Ser. No. 62/663,122 filed 26 Apr. 2018 (“Parent Provisional”).

The subject matter of this Application is also related to the subject matter of PCT Application Serial No. PCT/EP2014/060722, filed 23 May 2014 (“First Related Application”).

The subject matter of this application is related to U.S. application Ser. No. 15/375,739, filed 12 Dec. 2016 (“Second Related Application”).

This Application claims priority to:

-   -   1. the First Divisional Application;     -   2. the First Parent Application; and     -   3. the Parent Provisional;         collectively, “Related References”, and hereby claims benefit of         the filing dates thereof pursuant to 37 CFT § 1,78(a)(4).

The subject matter of the Related References, and the First and Second Related Applications, each in its entirety, is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to ultra-wideband communication systems, and, in particular, to a receiver for use in an ultra-wideband communication system adapted to determine the location of an RF transmitter relative to an RF receiver.

2. Description of the Related Art

In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art which should be familiar to those skilled in the art of ultrawideband (“UWB”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.

In the RF system topology shown in FIG. 1, it can be seen that, because of the nonzero angle of incidence, θ, the RF signal will arrive at one antenna before the other. In particular, it can be seen that the path to antenna A is greater than to antenna B by p=d*sin θ. In order to calculate θ, the angle of incidence, the time difference of arrival could be found. If d is relatively large then this would provide quite an accurate estimate of θ. On the other hand, if d is small the estimate turns out to be highly error prone.

FIG. 2 shows two receivers, 10 a and 10 b, which are clocked from the same crystal 12. If the same crystal 12 clocks identical phase locked loops (“PLLs”), 14 a and 14 b, the generated carriers that are supplied to the respective down converter mixers, 16 ac-16 as and 16 bc-16 as, will have the same phase. The RF signal will arrive at a slightly later time at antenna A than antenna B, so it will encounter a down converter carrier phase that is different in each of the mixers 16. If the baseband processors, 18 a and 18 b, are capable of calculating the complex impulse response of the channel, that impulse response will have a different in phase (“I”) to quadrature (“Q”) ratio I/Q which is equal to the phase delay caused by the signal travelling the extra distance, p, before encountering the mixer 10 a and being down-converted by the carrier. If the carrier frequency is high, e.g., 4 GHz or 6.5 GHz, then quite small distances, p, will lead to a relatively large carrier phase difference.

sinθ=P/d   [Eq. 1]

λ=c/f   [Eq. 2]

-   -   where:     -   f is the carrier frequency,     -   c is the speed of light, and     -   λ is the carrier wavelength.

α/2π=P/λ  [Eq. 3]

-   -   where:     -   αis the phase difference between the two carriers for the same         point on the incident RF signal.

(from Eq. 2 and Eq. 3)

p=αλ/2π  [Eq. 4]

(from Eq. 1 and Eq. 4)

sinθ=αλ/2πd   [Eq. 5]

(from Eq. 5)

θ=sin⁻¹(αλ/2πd)   [Eq. 6]

Many angle-of-arrival (“AoA”) estimators require the use of multiple receivers, each listening to its own antenna, see, e.g., the First Related Application. In some AoA estimators, a single receiver is electronically switched between a plurality of antennas, see, e.g., the Second Related Application. In all such embodiments, the AoA approach tends to be problematic. In the Second Related Reference, several possible solutions are proposed.

Using two or more receiver antennas, the location of a transmitter can be found by using time of flight to get the distance to the transmitter and using the difference in either; time of arrival, or phase of arrival, to calculate an angle of arrival. However, we submit that it is unnecessary to calculate the angle of arrival in order to find that location. Further, we submit that what is needed is an improved method and apparatus for use in the receiver of a UWB communication system to determine the (x,y) location of a transmitter with respect to a receiver. In particular, we submit that such a method and apparatus should provide performance generally comparable to the best prior art techniques but more efficiently than known implementations of such prior art techniques.

BRIEF SUMMARY OF THE INVENTION

In accordance with a one embodiment of our invention, we provide a method for determining the (x,y) location of a UWB transmitter relative to a UWB receiver. In accordance with this method, we use: the first antenna to receive a transmitted signal; the second antenna to receive the transmitted signal; and the receiver to: develop a range, r, between the transmitter and the first antenna; develop a first phase value as a function of the complex baseband impulse response of the transmitted signal received by the first antenna; develop a second phase value as a function of the complex baseband impulse response of the transmitted signal received by the second antenna; develop a phase difference value, p, as a function of the first and second phase values; and develop the (x,y) location of the transmitter relative to the receiver as a function of d, r and p.

In accordance with another embodiment of our invention, we provide a method for determining the (x,y) location of a UWB transmitter relative to a UWB receiver. In accordance with this method, we use: the first antenna to receive a transmitted signal; the second antenna to receive the transmitted signal; and the receiver to: develop a range, r, between the transmitter and the first antenna; develop a first time of arrival value as a function of the complex baseband impulse response of the transmitted signal received by the first antenna; develop a second time of arrival value as a function of the complex baseband impulse response of the transmitted signal received by the second antenna; develop a path difference value, p, as a function of the first and second time of arrival values; and develop the (x,y) location of the transmitter relative to the receiver as a function of d, r and p.

In accordance with yet another embodiment of our invention, we provide a method for determining the (x,y) location of a UWB transmitter relative to a UWB receiver. In accordance with this method, we use: the first antenna to receive a transmitted signal; the second antenna to receive the transmitted signal; and the receiver to: develop a range, r, between the transmitter and the first antenna; develop a first time of flight value as a function of the complex baseband impulse response of the transmitted signal received by the first antenna; develop a second time of flight value as a function of the complex baseband impulse response of the transmitted signal received by the second antenna; develop a path difference value, p, as a function of the first and second time of flight values; and develop the (x,y) location of the transmitter relative to the receiver as a function of d, r and p.

In accordance with one other embodiment of our invention, we provide a method for determining the (x,y) location of a UWB transmitter relative to a UWB receiver. In accordance with this method, we first use: the first antenna to receive a first transmitted signal; the second antenna to receive the first transmitted signal; and the receiver to: develop a first range, r₁, between the transmitter and either the first or second antenna; develop a first phase value as a function of the complex baseband impulse response of the first transmitted signal received by the first antenna; develop a second phase value as a function of the complex baseband impulse response of the second transmitted signal received by the second antenna; develop a first phase difference value, p₁, as a function of the first and second phase values. Then, we use: the first antenna to receive a second transmitted signal; the second antenna to receive the second transmitted signal; and the receiver to: develop a second range, r₂, between the transmitter and either the first or second antenna; develop a third phase value as a function of the complex baseband impulse response of the second transmitted signal received by the first antenna; develop a fourth phase value as a function of the complex baseband impulse response of the second transmitted signal received by the second antenna; develop a second phase difference value, p₂, as a function of the third and fourth phase values; and develop the (x,y) location of the transmitter relative to the receiver as a function of d, r₁, p₁, r₂ and p₂.

In one embodiment, we provide a location determination circuit configured to perform our method for determining relative location.

In one other embodiment, we provide an RF receiver comprising a location determination circuit configured to perform our method for determining relative location.

In yet another embodiment, we provide an RF transceiver comprising an RF receiver comprising a location determination circuit configured to perform our method for determining relative location.

In still another embodiment, we provide an RF communication system comprising an RF transceiver comprising an RF receiver comprising a location determination circuit configured to perform our method for determining relative location.

The methods of our invention may be embodied in non-transitory computer readable code on a suitable computer readable medium such that when a processor executes the computer readable code, the processor executes the respective method.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Our invention may be more fully understood by a description of certain preferred. embodiments in conjunction with the attached drawings in which:

Fig. 1 illustrates, generally in topographic perspective, an RF communication system, and, in particular, illustrates the different angles of incidence of the transmitted RF signal on two antennas spaced apart by a distance d;

FIG. 2 illustrates, in block diagram form, the antennas of FIG. 1, together with the respective RF receivers;

FIG. 3 illustrates, in block diagram form, one application of our invention to determine the location of a transmitter relative to a multi-antenna receiver; and

FIG. 4 illustrates, in chart form, the relationship between range and phase difference for an antenna separation of λ/2 using a single carrier;

FIG. 5 illustrates, in flow diagram form, a phase difference embodiment of a method adapted for use in the embodiment set forth in FIG. 3;

FIG. 6 illustrates, in flow diagram form, a time of arrival difference embodiment of a method adapted for use in the embodiment set forth in FIG. 3;

FIG. 7 illustrates, in flow diagram form, time of flight difference embodiment of a method adapted for use in the embodiment set forth in FIG. 3;

FIG. 8 illustrates, in chart form, the relationship between range and phase difference for an antenna separation greater than λ/2 using a single carrier;

FIG. 9 illustrates, in chart form, the relationship between range and phase difference for the same antenna separation as in FIG. 8, i.e., greater than λ/2, but using a different carrier than in FIG. 8;

FIG. 10 illustrates, in chart form, one example of a correction function for compensating for path distortion resulting from real world effects; and

FIG. 11, comprising FIG. 11A and FIG. 11B, illustrates, in flow diagram form, a multi-carrier, multi-transmission embodiment of a method adapted for use in the embodiment set forth in FIG. 3.

In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that our invention requires identity in either function or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that it is possible to get the Cartesian (x,y) location of a transmitter relative to a multi-antenna receiver directly from the range and the path difference without going through an intermediary step of calculating an angle of arrival. In accordance with our invention, the path difference can be determined either by using: the difference in the phase of a received frame at two of the antennas; the difference in time of arrivals of a received frame at two of the antennas; or the difference in ranges measured to two of the antennas. Further, our method can be implemented in an RF system comprising: multiple receivers, each with a respective antenna; a single receiver having multiple antennas; or any combination thereof.

In the example illustrated in FIG. 3, the geometric distance between antenna A and antenna B is known to be d; and the path difference between antenna A and antenna B is determined to be p. Using well known methods, we can determine the range, r, between antenna A and antenna C, e.g., by measuring the time of flight between A and C. Provided that d is less than or equal to one half the wavelength, λ, of the signal transmitted from antenna C, p will always be between −λ/2 and +λ/2. We can now determine the (x,y) location of the transmitter relative to the receiver as follows:

using the cosine rule:

cos(A)=(b ² +c ²−α²)/2bc   [Eq. 7]

cos(α)=(r ² +d ²−(r−p)²)/2rd   [Eq. 8]

x/r=(r ² +d ² −r ²+2rp−p ²)2rd   [Eq. 9]

x=(d ²+2rp−p ²)2d   [Eq. 10]

r ² =x ² +y ²   [Eq. 11]

y=±√{square root over (r ² −x ²)}  [Eq. 12]

or, alternatively:

(x ² +y ²)=((d ²+2rp−p ²)/2d)² +y ² =r ²   [Eq. 13]

y ² =r ²−((d ²+2rp−p ²)/2d)²   [Eq. 14]

y=±(√{square root over ((d ² −p ²)((2r−p)² −d ²))}/2d)   [Eq. 15]

y=±(√{square root over ((d ² −p ²)(4r ²−4rp+p ² −d ²))}/2d)   [Eq. 16]

y=±(√{square root over ((1−(p/d)²)(4r ²−4rp+p ² −d ²))}/2)   [Eq. 17]

Note that we cannot tell if the y coordinate is positive or negative, so there is a front/back ambiguity.

We note that d is small compared to r so d² is very small compared to r² and can be neglected:

y≈±(√{square root over ((1−(p/d)²)(4r ²−4rp+p ²))}/2)   [Eq. 18]

y≈±(r−(p/2))√{square root over (1−(p/d)²)}  [Eq. 19]

We have determined that the maximum error by using this approximation is 0.22 mm for a 6.5 GHz carrier and a receiver antenna separation of λ/2. So, using Eq. 10 and Eq. 12, or Eq. 10 and Eq. 19, we can calculate the (x, y) coordinates of the transmitter without calculating the angle of arrival. We just need to know:

-   -   r—the range to one of the antennas;     -   d—the distance between the two antennas; and     -   p—the path difference for the signal arriving at the two         antennas.

One of the most accurate ways to get the path difference is to get the phase difference of arrival of the transmitted signal in fractions of a cycle, and then multiply by the wavelength. Another way is to get the time difference of arrival of the transmitted signal, and multiply by the speed of light. A third way is to get the difference in time of flight, and multiply by the speed of light.

As can be seen in FIG. 4, however, that the location uncertainty at phase differences near +/−180° is quite large. Thus, a very small change in phase gives a large change my position. We can see this sensitivity in Eq. 19, resulting from the (1−(p/d)²) term under the radical. Accordingly, small errors due to noise at these larger phase differences are amplified compared to when the phase difference is in the +/−90° region. Also, extra care must be taken when calculating the square root in this region.

Consider, for example, the Taylor series for √{square root over (1−x ²)}:

$1 - \frac{x^{2}}{2} - \frac{x^{4}}{8} - \frac{x^{6}}{16} - \frac{5x^{8}}{128} - \frac{7x^{10}}{256} - \frac{21x^{12}}{1024} - \frac{33x^{14}}{2048} - \frac{429x^{16}}{32768} + {O\left( x^{18} \right)}$ (Taylorseries)

Using 6 terms in this Taylor expansion, the calculation is accurate to 2 cm up to a phase difference of 140° and to 5 cm up to 150°. Above 150°, however, the error increases rapidly. So, we will use a two-polynomial piece-wise approximation for the square root function:

function y=polysqrt(x);  p1=[107.81 −33.530 4.91940 0.05019]; % polynomial 1 coefficients  p2=[0.3557 −0.9585 1.40389 0.20150]; % polynomial 2 coefficients  y1=sum(p1.*x.{circumflex over ( )}[3 2 1 0]); % evaluate y1 for small x  y=sum(p2.*x.{circumflex over ( )}[3 2 1 0]); % evaluate y for larger x  y(x<0.144)=y1(x<0.144); % use y1 fox x<0.144  y(x<.002)=0; % set very small values to zero end Using this approximation gives a worst case error in position of 2.5 cm within a 10 m radius, and an RMS error of 0.77 cm.

By way of example, in FIG. 5, we have illustrated a phase difference embodiment of our method adapted for use in the embodiment illustrated in FIG. 3. Further, also by way of example, we have illustrated in FIG. 6 a time of arrival difference embodiment of our method adapted for use in the embodiment illustrated in FIG. 3. In FIG. 7, we have illustrated one example of a time of flight difference embodiment of our method adapted for use in the embodiment illustrated in FIG. 3.

When the distance between the antennas is greater than half the wavelength of the center frequency, a particular phase difference can give more than one valid solution, because adding 360° will give the same phase difference. For instance, if the center frequency is 8 GHz and the distance between the antennas is 1 wavelength of a 6.5 GHz carrier, we would have the situation shown in FIG. 8. As we can see, for many values of phase difference, there are three possible solutions. But switching to a channel with a 6.5 GHz carrier would give the situation shown in FIG. 9. In this case, apart from the 0° case, there are always two possible solutions for each phase difference. Fortunately, we can use this fact to our advantage. If the distance between the antennas is greater than one half wavelength, we can set the carrier frequency to one channel, and then measure the range and phase difference as before. This will give us more than one (x, y) coordinate—let us call this set of possible solutions Set 1. We then switch the carrier frequency to a different channel, and repeat the measurement on the new channel. This will also give us multiple solutions—let us call this set of possible solutions Set 2. We can then discard all solutions which don't appear in both Set 1 and Set 2. This will generally leave only one (,y) coordinate. By way of example, in FIG. 11, we have illustrated one example of a multi-carrier, multi-transmission embodiment of our method adapted for use in the embodiment illustrated in FIG. 3.

In practice, noise perturbations in the phase measurement will almost always ensure that the corresponding solutions will not be exactly the same, so we must pick pairs of solutions, one from each set, which are closest to each other in the Euclidean sense. A well known way to do this is to pick the (x_(i),y_(i)) from Set 1 and the (x_(j),y_(j)) from Set 2 that minimizes:

√{square root over ((x _(i) +x _(j))²+(y _(i) −y _(j))²)}  [Eq. 20]

To get even better results, we can repeat this on a third (or even fourth or fifth) channel. For a third channel, we could pick the (x_(i),y_(i)) from Set 1, (x_(j),y_(j)) from Set 2 and the (x_(k),y_(k)) from a Set 3 that minimizes:

$\begin{matrix} \sqrt{\begin{matrix} {\left( {x_{i} - x_{j}} \right)^{2} + \left( {x_{i} - x_{k}} \right)^{2} + \left( {x_{j} - x_{k}} \right)^{2} +} \\ {\left( {y_{i} - y_{j}} \right)^{2} + \left( {y_{i} - y_{k}} \right)^{2} + \left( {y_{j} - y_{k}} \right)^{2}} \end{matrix}} & \left\lbrack {{Eq}.21} \right\rbrack \end{matrix}$

Up to this point, we have assumed that the effective, i.e., measured, path difference is precisely equal to the Cartesian path difference. However, in a real system, the effective path difference and the Cartesian path difference will not be precisely equal. For example, when our two antennas are closer than a few wavelengths, the antennas interact through an effect known as mutual coupling; this causes the electromagnetic waves to behave differently than would be the case in free space resulting in different effective and Cartesian path differences as a function of the path difference. Another effect seen in the real world is that the feed wires to the antennas can have slightly difference lengths, or the paths from the down-mixer generator to the two separate down-mixers can have slightly different lengths. These two effects, and others, add a calculatable offset to the path difference.

We propose to compensate for these cumulative effects by developing a Calibration Function between the effective and Cartesian path differences. In accordance with this embodiment, we first perform a system calibration process wherein a number of calibration measurements are taken, each from a different known (x, y) coordinate but, collectively, having a wide range of actual path differences. Since we know all of the (x, y) coordinates, we can calculate the respective expected path differences. By measuring the respective phase differences, we can also calculate the respective effective path differences. In this way we can develop a suitable Calibration Function, e.g., in the form of a look-up table or as a piece-wise linear generator function or by using a polynomial fitting function. Then we can use this Calibration Function to correct the effective path difference, p, before applying the formulas to calculate the (x, y) coordinates. By way of example, in FIG. 10, we have illustrated one possible Calibration Function.

Although we have described our invention in the context of particular embodiments, one of ordinary skill in this art will readily realize that many modifications may be made in such embodiments to adapt either to specific implementations. For example, rather than calculating the path difference, p, one might directly determine the range, r₂, between the second antenna and the receiver using the same method used to determine the range, r₁, between the first antenna and the receiver. Further, the several elements described above may be adapted so as to be operable under either hardware or software control or some combination thereof, as is known in this art. Alternatively, the several methods of our invention as disclosed herein in the context of special purpose receiver apparatus may be embodied in computer readable code on a suitable non-transitory computer readable medium such that when a general or special purpose computer processor executes the computer readable code, the processor executes the respective method.

Thus it is apparent that we have provided a method and apparatus for determining the location of a wireless transmission relative to a multi-antenna receiver. Although we have so far disclosed our invention only in the context of a packet-based UWB communication system, we appreciate that our invention is broadly applicable to other types of wireless communication systems, whether packed-based or otherwise, that perform channel sounding. Further, we submit that our invention provides performance generally comparable to the best prior art techniques but more efficiently than known implementations of such prior art techniques. 

1-8. (Canceled)
 9. A method comprising: developing a range between a transmitter of a transmitted signal and one of: a first antenna of a receiver that received the transmitted signal; or a second antenna of the receiver that received the transmitted signal; developing a first phase value as a function of a complex baseband impulse response of the transmitted signal received at the first antenna; developing a second phase value as a function of a complex baseband impulse response of the transmitted signal received at the second antenna; developing a path difference value as a function of the first phase value and the second phase value; and developing a location of the transmitter relative to the receiver as a function of the range, the path difference value, and a distance between the first antenna and the second antenna.
 10. The method of claim 9, further comprising: receiving the transmitted signal from the first antenna of the receiver; and receiving the transmitted signal from the second antenna of the receiver.
 11. The method of claim 9, wherein the distance is less than or equal to one half a wavelength of the transmitted signal.
 12. The method of claim 9, wherein developing the path difference value as the function of the first phase value and the second phase value comprises developing the path difference value as a function of the first phase value and the second phase value and a calibration function.
 13. A non-transitory computer readable medium including executable instructions, that when executed by a processor, cause a system to: develop a range between a transmitter of a transmitted signal and one of: a first antenna of a receiver that received the transmitted signal; or a second antenna of the receiver that received the transmitted signal; develop a first phase value as a function of a complex baseband impulse response of the transmitted signal received at the first antenna; develop a second phase value as a function of a complex baseband impulse response of the transmitted signal received at the second antenna; develop a path difference value as a function of the first phase value and the second phase value; and develop a location of the transmitter relative to the receiver as a function of the range, the path difference value, and a distance between the first antenna and the second antenna.
 14. The non-transitory computer readable medium of claim 13, wherein the non-transitory computer readable medium further includes executable instructions, that when executed by the processor, cause the system to: receive the transmitted signal from the first antenna of the receiver; and receive the transmitted signal from the second antenna of the receiver.
 15. The non-transitory computer readable medium of claim 13, wherein developing the path difference value as the function of the first phase value and the second phase value comprises developing the path difference value as a function of the first phase value and the second phase value and a calibration function.
 16. The non-transitory computer readable medium of claim 13, wherein the system is an RF receiver.
 17. The non-transitory computer readable medium of claim 13, wherein the system is an RF transceiver.
 18. The non-transitory computer readable medium of claim 13, wherein system is an RF communication system.
 19. The non-transitory computer readable medium of claim 13, wherein the system is a location determination circuit.
 20. The non-transitory computer readable medium of claim 13, wherein the distance is less than or equal to one half a wavelength of the transmitted signal.
 21. A device, comprising: a processor; and a non-transitory computer readable medium including executable instructions, that when executed by the processor, cause the device to: develop a range between a transmitter of a transmitted signal and one of: a first antenna of a receiver that received the transmitted signal; or a second antenna of the receiver that received the transmitted signal; develop a first phase value as a function of a complex baseband impulse response of the transmitted signal received at the first antenna; develop a second phase value as a function of a complex baseband impulse response of the transmitted signal received at the second antenna; develop a path difference value as a function of the first phase value and the second phase value; and develop a location of the transmitter relative to the receiver as a function of the range, the path difference value, and a distance between the first antenna and the second antenna.
 22. The device of claim 21, wherein the non-transitory computer readable medium further includes executable instructions, that when executed by the processor, cause the device to: receive the transmitted signal from the first antenna of the receiver; and receive the transmitted signal from the second antenna of the receiver.
 23. The device of claim 21, wherein developing the path difference value as the function of the first phase value and the second phase value comprises developing the path difference value as a function of the first phase value and the second phase value and a calibration function.
 24. The device of claim 21, wherein the device is an RF receiver.
 25. The device of claim 21, wherein the device is an RF transceiver.
 26. The device of claim 21, wherein the device is an RF communication system.
 27. The device of claim 21, wherein the device is a location determination circuit.
 28. The device of claim 21, wherein the distance is less than or equal to one half a wavelength of the transmitted signal. 